Less-than operator causes error "not well-formed" in xsl-if

I'm going through the w3cschools XSLT tutorial, and I am at this page: xsl-if.

On that page (in red) is the text <xsl:if test="price &gt; 10">. This works. I modified the code to use "<" and that works fine too.

I tested <xsl:if test="price > 10"> (note the use of > instead of the >). This works too.

But this fails: <xsl:if test="price < 10">. Error is XML Parsing Error: not well-formed and it points to the < symbol in the expression.

If the > symbol worked fine, why did using the < fail? (I'm using FireFox)